LocalPath.IsPrefixOf: account for leading .. references

[rghvgrv]

Closes #226.

IsPrefixOf compared relative paths as strings, which breaks once a path begins with ..:

new LocalPath("..").IsPrefixOf(new LocalPath("foo"));   // was false, now true
new LocalPath("..").IsPrefixOf(new LocalPath("../.."));  // was true, now false

A relative path is now split into the number of its leading .. references and the remaining segments:

• same number of .. references: the remaining segments are compared as before (whole-segment, platform-default case sensitivity);
• the prefix starts higher up: it's a prefix only if it is a bare train of .. references (.., ../.., …); otherwise it names a directory that is only known after resolving the current directory, so the result is false;
• the prefix starts lower down: never a prefix.

This also covers the empty-path special case from #223, so that branch is folded into the general rule. Absolute paths are compared exactly as before.

Not covered here: /.. on rooted paths (mentioned in the issue). It needs a decision on whether normalization should drop .. directly after the root, so I've left it for a separate change.

Tests: added cases to LocalPathTests.IsPrefixOfAndStartsWith; the full suite passes locally.